Turbine cascade wind tunnels simulate Reynolds number and turbulence for the examination of flow phenomena such as boundary layer separation on the trailing portion of low pressure turbine blades. Separation may occur for axial chord-based Reynolds numbers of about 50 k to about 100 k Axial chord-based Reynolds numbers may be from about 50 k to about 100 k. Various passive, square-bar turbulence generating grid arrangements may be explored to simulate turbulence in a turbine cascade test-section inlet, including two grid orientations: perpendicular to the air inlet flow and parallel to the turbine cascade. A novel T-bar grid configuration oriented parallel to a turbine blade cascade was shown to produce improved test section inlet flow field uniformity than that produced by a perpendicular mesh grid. Improved periodicity in blade-to-blade surface pressure coefficient profiles may also observed with the parallel T-Bar grid.
Turbine cascade wind Tunnels are similar to conventional wind tunnels except the test section of interest is in a corner. FIG. 5 shows an illustration of a closed-loop cascade wind tunnel. Cascade wind tunnel facilities are used to simulate turbine operating conditions for the study of flow phenomena such as boundary layer separation over the trailing portion of a turbine suction surface. In addition to Reynolds number, turbulence is preferably simulated as well because turbine performance is sensitive to both. Turbines are susceptible to adverse flow effects at low Reynolds numbers and low turbulence intensities.
It is possible to evaluate various turbulence generating grid configurations in a cascade wind tunnel.
Different designs for turbulence generating grids have been proposed for many years, including U.S. application Ser. No. 12/897,105; and U.S. application Ser. No. 12/296,004. Further complicating the generation of turbulent flow is having it applied uniformly across a turbine blades in a cascade wind tunnel. Conventional turbulence generating grids designs have two limitations. First, they are not designed or optimized for the corner test section of a curved cascade wind tunnel where the turbine blades, unlike many other structures are at an angle to the air flow. When the grid assembly inserted perpendicular to the air flow, the turbulence exits the turbulence generating grids perpendicular to the turbulence generating grids and air flow, not the turbine blades being modeled. A cursory review to air flow turbulence generation may conclude that the standard grid, placed in the air flow at the same angle as the turbine blades, would obviously solve this problem. It does not. The prior art turbulence generating grids angled to the flow redirect the flow so that the angle of attack experienced by the turbines is changed from their original design, making measurements even less useful than with a perpendicular turbulence generating grid in many cases. The present invention is an advancement which reduces this deficiency in the prior art with an improved T-bar grid design for cascade wind tunnels, reducing the undesirable changes in air flow. The T-bar grid design allows the overall assembly to be angled to the flow while individual turbulence grid elements are perpendicular to the flow.